Method for concentrating oxygen isotope

ABSTRACT

A method of concentrating the stable oxygen isotopes of  17 O and  18 O by irradiating ozone with light, selectively dissociating an isotopologue of ozone containing an oxygen isotope in its molecule into oxygen, followed by dissociating the ozone and separating the formed oxygen from the non-dissociated ozone. 
     In the ozone photodissociation step, light is radiated onto a rare gas-ozone mixed gas containing ozone and at least one rare gas selected from krypton, xenon and radon is used to selectively dissociate ozone containing a specific oxygen isotope in its molecule into oxygen then the oxygen isotope is separated from non-dissociated ozone and rare gas to concentrate the oxygen isotope present in the separated oxygen.

This application is the U.S. national phase of international applicationPCT/JP2004/002568, filed 2 Mar. 2004, which designated the U.S. andclaims priority of JP 2003-057439, filed 4 Mar. 2003; JP 2003-200951,filed 24 Jul. 2003; JP 2003-319729, filed 11 Sep. 2003, the entirecontents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for concentrating oxygenisotopes, and more particularly, to a method for selectivelyconcentrating the stable oxygen isotopes, ¹⁷O and/or ¹⁸O, which exist inextremely small natural abundance, by using an ozone photodissociationreaction or a peroxide photodissociation reaction.

2. Description of the Related Art

Since ¹⁷O and ¹⁸O, which are oxygen isotopes used as tracers in thefields of chemistry and medicine, exist in extremely small naturalabundance, it is necessary that they be concentrated. A known method forconcentrating ¹⁸O consists of obtaining a product that contains ¹⁸O byselectively photodissociating a saturated chain ether that contains ¹⁸Oby irradiating it with laser light (refer to, for example, JapaneseExamined Patent Application, Second Publication No. 6-102134). Othermethods include a method for selectively concentrating ¹⁸O byirradiating formaldehyde or carbon monoxide with laser light.

However, for reasons such as a low concentration rate, low utilizationefficiency of the light that contributes to the reaction, lowluminescence efficiency of the laser light and the necessity forpost-processing for extracting the concentrated chemical species, themethods of the prior art have not been established industrially.Although the concentration of oxygen isotopes using a distillationprocedure has also been proposed, problems are encountered in order toincrease the degree of concentration, such as increased size of thedevice and the start-up time of the device being extremely long.

Therefore, the object of the present invention is to provide a methodfor concentrating oxygen isotopes that are capable of concentrating aoxygen isotope either in the state of the stable oxygen isotopes of ¹⁷Oor ¹⁸O, or in a substance such as water having a simple molecularstructure.

SUMMARY OF THE INVENTION

In order to achieve the aforementioned object, the oxygen isotopeconcentration method of the present invention comprises (a) an ozonephotodissociation step, in which ozone molecules containing oxygenisotopes, ¹⁷O and/or ¹⁸O, are selectively photodissociated to oxygenmolecules; thereby generating a mixture of the oxygen molecules andnon-dissociated ozone molecules, followed by (b) an oxygen isotopeconcentration step, in which the oxygen molecules are separated from themixture, thereby concentrating the oxygen isotope. More specifically,the present invention comprises (A) an ozone formation step, in whichozone is produced from raw material oxygen, thereby forming a mixture ofthe produced ozone and the raw material oxygen, (B) an ozone separationstep, in which the produced ozone is separated from the mixture, (C) anozone photodissociation step, in which ozone molecules containing oxygenisotopes, ¹⁷O and/or ¹⁸O, are selectively photodissociated to oxygenmolecules; thereby generating a mixture of the oxygen molecules andnon-dissociated ozone molecules, and (D) an oxygen isotope concentrationstep, in which the oxygen molecules are separated from the mixtureobtained in the step (C), thereby concentrating the oxygen isotope.

Different oxygen isotopes can also be concentrated in two stages bycarrying out (E) a second ozone photodissociation step, in which thenon-dissociated ozone molecules containing an oxygen isotope that isdifferent from one separated in the step (D) are photodissociated tooxygen molecules, thereby generating a mixture of the oxygen moleculesand non-dissociated ozone molecules, and (F) a second oxygen isotopeconcentration step, in which the oxygen molecules are separated from themixture obtained in the step (E), thereby concentrating the oxygenisotope that is different from one separated in the step (D).

The oxygen isotope concentration method of the present invention mayalso contain an oxygen isotope concentration step in which ozone isirradiated with light after adding at least one type of krypton, xenonor radon rare gas in the photodissociation step (a) or (B), therebygenerating a mixture of the oxygen molecules, non-dissociated ozonemolecules, and the rare gas; and the oxygen molecules obtained in theozone photodissociation step is separated from the mixture.

In the oxygen isotope concentration method of the present invention, theoxygen isotope concentration step (b) or (D) may be distillation inwhich at least one type of helium, neon or argon rare gas is added forthe oxygen isotope concentration step.

In the present invention, at least one type of rare gas selected fromhelium, neon, argon or krypton is added to the raw material oxygen inthe step (A).

In the present invention, (G) an ozone abatement step, in which thenon-dissociated ozone molecules remaining after the step (D) aredecomposed into oxygen molecules, thereby forming a mixture of theoxygen molecules and the rare gas, and (H) a rare gas separation step,in which the rare gas is separated from the mixture obtained in the step(G), are carried out, wherein the rare gas separated in the step (H) isrecycled for use in the step (A).

In the present invention, (E′) a second ozone photodissociation step, inwhich molecules of an isotopologue of ozone that are different fromthose of the isotopologue of ozone dissociated in the step (C) arephotodissociated to oxygen molecules, thereby generating a mixture ofthe oxygen molecules and non-dissociated ozone molecules, and (F′) asecond oxygen isotope concentration step, in which the oxygen moleculesobtained in the step (E′) are separated from the mixture generated inthe step (E′), thereby concentrating the second oxygen isotope in theseparated oxygen molecules.

In the oxygen isotope concentration method of the present invention, thelight used in the ozone photodissociation step (a) or (C) is preferablyeither light within a range of 700-1000 nm, or light within a range of450-850 nm. In particular, the wavelength of the light used in the ozonephotodissociation step is more preferably within the range of 991.965 to992.457 nm. It is also preferable to adjust the absorption wavelength ofozone by applying an electric field in the ozone photodissociation step(a) or (C). It is also preferable to carry out the ozonephotodissociation step (a) or (C) at low temperature and low pressure.

There are 18 types of the aforementioned ozone isotopologues, consistingof ¹⁶O¹⁶O¹⁶O, ¹⁶O¹⁶O¹⁷O, ¹⁶O¹⁷O¹⁶O, ¹⁶O¹⁶O¹⁸O, ¹⁶O¹⁸O¹⁶O, ¹⁶O¹⁷O¹⁷O,¹⁷O¹⁶O¹⁷O, ¹⁶O¹⁷O¹⁸O, ¹⁷O¹⁶O¹⁸O, ¹⁶O¹⁸O¹⁷O, ¹⁷O¹⁷O¹⁷O, ¹⁶O¹⁸O¹⁸O,¹⁸O¹⁶O¹⁸O, ¹⁷O¹⁷O¹⁸O, ¹⁷O¹⁸O¹⁷O, ¹⁷O¹⁸O¹⁸O, ¹⁸O¹⁷O¹⁸O and ¹⁸O¹⁸O¹⁸O.

In the present invention, the oxygen molecules containing a desiredoxygen isotope, ¹⁷O or ¹⁸O can be photodissociated by irradiating ozonemolecules containing a mixture of these various types of isotopologueswith light.

For example, when the isotopomer ¹⁶O¹⁶O¹⁷O is irradiated with light,three molecules of oxygen are generated from two molecules of ozoneaccording to the reaction formulas shown below.¹⁶O¹⁶O¹⁷O+“irradiated with light”→O₂+O  (1)O₃+O→2O₂  (2)

In this reaction, the ¹⁷O that composed the ozone that wasphotodissociated in reaction formula (1) is either contained in theformed “O₂” or is in the form of “O”. Since this “O” immediately reactswith other ozone to form two molecules of oxygen as shown in reactionformula (2), this means that the ¹⁷O is present in one of the threemolecules of oxygen formed in reaction formulas (1) and (2). Althoughthere is the possibility of ¹⁷O or ¹⁸O also being contained in the ozonethat reacts in reaction formula (2), the probability is extremely lowsuch that the amount can be ignored.

The bond dissociation energy of ozone is 1.05 eV, and ozone breaks downduring absorption of light having a wavelength of 1.18 μm or less. Thislight absorption by ozone is known to take place in the bands indicatedbelow.

Wulf band 700-1000 nm (1.2-1.8 eV) Near infrared band

Chappius band 450-850 nm (1.5-2.8 eV) Visible band

Huggins band 300-360 nm (3.4-4.1 eV) Ultraviolet band

Hartly band 200-300 nm (4.1-6.2 eV) Ultraviolet band

Even in these bands, in the vicinity of a wavelength of 1000 nm (wavenumber: 10000 cm⁻¹) of the Wulf band, a sharp absorption peak as shownin FIG. 1 is observed according to the literature (Journal of ChemicalPhysics, Vol. 108, No. 13, p. 5449-5475). FIG. 1 shows the opticalabsorbance for ¹⁶O₃(¹⁶O¹⁶O¹⁶) and ¹⁸O₃(^(18 O) ¹⁸O¹⁸O). The maximum peakof ¹⁶ O₃ can be seen to be at a wave number of 10081 cm⁻¹ (wavelength:991.965 nm), while the maximum peak of ¹⁸O₃ can be seen to be at a wavenumber of 10076 cm⁻¹ (wavelength: 992.457 nm). Thus, the wavelength atwhich an isotopologue of ozone containing ¹⁷O or ¹⁸O can be efficientlydissociated is located between these two, and it can be seen that thedesired ozone can be selectively dissociated by selecting a wavelengthwithin that range.

Although other absorption peaks can be used, in consideration of thedissociation efficiency of the ozone, the aforementioned range isoptimal. In particular, there is also the advantage of such light in thenear infrared or visible band being easier to handle in comparison withthe case of using light of the ultraviolet band. If ultraviolet light ofa high energy level is used, in addition to the target ozoneisotopologue, other ozone isotopologues may also end up beingdissociated, thereby lowering the concentration efficiency of the oxygenisotope.

In the case of poor selective dissociation efficiency due to the lightsource having shifted slightly from the desired ozone dissociationwavelength, since the absorption wavelength of ozone can be shiftedusing the Stark effect by applying an electric field to the ozone whenirradiating with light, the absorption wavelength of the ozone can bemade to precisely match the wavelength of the light source.

Examples of light sources that can be used to obtain light of thiswavelength include spectral light of sunlight as well as colored laserlight capable of optical pumping with an InGaAsP semiconductor laser orlight emitting diode, AlGaInP semiconductor laser or light emittingdiode, GaAsSb semiconductor laser or light emitting diode, CdZnTesemiconductor laser or light emitting diode, CdZnSe semiconductor laseror light emitting diode, mercury lamp, YAG laser, Ar ion laser or Kr ionlaser.

When irradiating ozone with light, light is preferably radiated at a lowpressure of, for example, 13 kPa (100 Torr) or less in order to lengthenthe mean free path of the ozone molecules and suppress molecularcollisions as much as possible. As a result, increases in absorptionwidth of the light caused by molecular collisions can be avoided. Inorder to suppress spontaneous dissociation of ozone, it is preferable tocool not only during irradiation of the ozone with light, but the entiresystem as well, to a low temperature within the range of, for example,100-250 K. As a result, in addition to making the absorption peakssharper, the formation of oxygen by spontaneous dissociation can besuppressed, thereby making it possible to improve the concentration rateof oxygen containing oxygen isotopes.

Ozone can be formed easily by silent discharge of oxygen serving as thematerial with an ozonizer, or by irradiating with ultraviolet light froma mercury lamp and so forth. Although oxygen of high purity thatcontains as few impurities such as argon and nitrogen as possible ispreferably used for the raw material oxygen, if these impurities can beadequately separated when separating the ozone and oxygen, thenindustrial oxygen having a purity of about 99.5% can also be used forthe raw material oxygen.

Oxygen consisting of ¹⁷O or ¹⁸O concentrated with the concentrationmethod of the present invention as well as oxygen consisting of ¹⁷O or¹⁸O concentrated with other methods can be used for the raw materialoxygen.

Separation of ozone and oxygen or separation of oxygen, ozone and raregas can be easily carried out by low-temperature distillation utilizingthe difference in boiling points between them, or by low-temperatureadsorption utilizing their differences in adsorption to an adsorbentsuch as silica gel.

As shown, for example, in the system diagram of FIG. 2, the optimumdevice configuration for carrying out the present invention is providedwith an ozone formation unit 11 that forms ozone from raw materialoxygen GO, an ozone separation unit 12 that separates ozone OZ formedwith the ozone generation unit and raw material oxygen, an ozonephotodissociation unit 13 that radiates light L of a specific wavelengthonto the ozone separated by ozone separation unit 12 and selectivelydegrades ozone containing a specific oxygen isotope in its molecule intooxygen, and an oxygen separation unit 14 that separates the oxygen OCformed by degrading ozone in ozone photodissociation unit 13 andnon-dissociated ozone OZ to concentrate the desired oxygen isotope inthe oxygen.

The device configuration for carrying out the present invention can alsobe provided with a second ozone photodissociation unit 15, whichradiates light L onto ozone OZ separated by oxygen separation unit 14and selectively degrades ozone that contains in its molecule a differentoxygen isotope from the ozone dissociated by the ozone photodissociationunit 13 into oxygen, and a second oxygen separation unit 16, whichseparates oxygen OC2 formed by the dissociation of ozone in the secondozone photodissociation unit 15 and non-dissociated OZ2 to concentratean oxygen isotope in the oxygen.

The amount of raw material oxygen consumed can be reduced by providing aline by which oxygen GO separated by ozone separation unit 12 isrecycled and fed into ozone formation unit 11. Ozone photodissociationunit 13 is preferably provided with a cooling unit and a pressurereduction unit.

In the case of concentrating oxygen isotopes using ozone, although it ispreferable to use ozone having as high a purity as possible inconsideration of the radiation efficiency of the light and theconcentration efficiency, the use of highly pure ozone may result inspontaneous dissociation due to catalysis with metal surfaces havingcatalytic action. If oxygen not containing the desired oxygen isotope isformed in large amounts due to spontaneous dissociation of ozone, thisoxygen can cause a considerable decrease in the concentration rate inthe aforementioned oxygen isotope concentration step.

As another method of concentrating oxygen isotopes of the presentinvention, ozone concentration can be lowered by adding a suitableamount of a rare gas to the ozone in order to prevent in advance anydecreases in the concentration rate of oxygen isotopes caused byspontaneous dissociation of ozone.

Since rare gases (helium, neon, argon, krypton, xenon and radon) havehardly any effect on the photochemical reactions of ozone in the ozonephotodissociation step, a specific ozone can be still be selectivelydissociated even if the ozone is diluted with these rare gases. Inaddition, handling of the ozone can be made easier as compared withhighly pure ozone by diluting it with a rare gas.

Mixing of ozone and rare gas can be carried out with an arbitrary deviceof each step, and a rare gas suitable for each step should be added in asuitable amount. At this time, rare gases that solidify at lowtemperatures, rare gases that condense at locations where they should begaseous, and rare gases that vaporize at locations where they should bea liquid are unsuitable, and it is necessary to select rare gasesaccording to the operating pressure and operating temperature. In thecase of using low-temperature distillation separation for the oxygenisotope concentration step and ozone separation step, although it isnecessary to ensure that the ozone does not reach a high concentrationby containing a suitable amount of rare gas in the liquid phase on theozone side, in order to obtain the rising gas and falling liquidnecessary for the distillation procedure, a rare gas that isconcentrated on the gas phase side may be used in combination. In thecase of collecting and reusing the rare gas, it is preferable to selecta rare gas that separates easily from the oxygen in the rare gasseparation step. However, xenon and radon are not preferable for use inthe ozone formation step since they form unstable compounds by reactingwith oxygen that is caused by silent discharge with an ozonizer orirradiation with ultraviolet light from a mercury lamp and so forth.

The oxygen isotope concentration device for carrying out the presentinvention can be provided with an ozone photodissociation unit, whichradiates light onto a rare gas-ozone mixed gas containing ozone and atleast one type of rare gas selected from krypton, xenon and radon, andselectively degrades ozone containing a specific oxygen isotope in itsmolecule into oxygen, and an oxygen isotope concentration unit, whichseparates the oxygen separated from ozone in the ozone photodissociationunit from non-dissociated ozone to concentrate the oxygen isotopepresent in the separated oxygen.

Still another oxygen isotope concentration method of the presentinvention comprises photodissociation of peroxide with semiconductorlaser light, and thereby increasing the concentration of an oxygenisotope in a photoreaction product.

Examples of the peroxide include at least one type of organic peroxidesuch as hydroperoxides such as HOOH and (CH₃)₃COOH, (di)alkyl peroxidessuch as CH₃OOCH₃, C₂H₅OOC₂H₅ and (CH₃)₃OO(CH₃)₃, peroxyacids includingperacids such as HCOOOH and CH₃OOOH, (di)acyl peroxides such asCH₃OOOCOCH₃ and

peroxyesters such as CH₃OOOC(CH₃)₃, (CH₃)₂CHCOOOC(CH₃)₃ and(CH₃)₃C—COOOC(CH₃)₃, peroxycarbonates such as (CH₃)₃COOCOOCH(CH₃)₂,peroxydicarbonates such as

diperoxycarbonates such as i-PrO—COOOCOO-i-Pr(i-Pr: isopropyl group),peroxalates, cyclic peroxides, ozonides and endoperoxides such as

or at least one type of nitric ester such as methyl nitrate, and ethylnitrate, or nitrous ester such as methyl nitrite or ethyl nitrite.

The aforementioned peroxides may be diluted with at least one type ofsolvent selected from carbon tetrachloride, acetone, acetic acid,hexane, toluene and chlorofluorocarbons, or organic substances having adouble bond, and the aforementioned dissociation is predissociation.

As a more concrete example of this method, the method further comprises

(I) a vaporization step, in which a solution prepared by dilutingperoxide having a O—H bond, O—O bond, C—O bond, or C═O bond with asolvent is vaporized under reduced pressure, and then fed into aphotoreaction cell;

(II) a photodissociation step, in which the peroxide in thephotoreaction cell is irradiated with semiconductor laser light havingphoton energy enough to dissociate the peroxide and wavelength of whichcorresponding to one of the absorption spectra of a O—H bond, O—O bond,C—O bond or C═O bond, thereby photodissociating the peroxide; and

(III) a purification step of the photoreaction product from the step(II), thereby concentrating ¹⁷O or ¹⁸O in one type of molecular species.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the absorption spectrum of ozone.

FIG. 2 is a system diagram showing an example of the configuration of adevice for carrying out the method of the present invention.

FIG. 3 is a system diagram showing a first embodiment of an oxygenisotope concentration device of the present invention.

FIG. 4 is a system diagram showing a second embodiment of an oxygenisotope concentration device of the present invention.

FIG. 5 is a system diagram showing a third embodiment of an oxygenisotope concentration device of the present invention.

FIG. 6 is a system diagram showing a fourth embodiment of an oxygenisotope concentration device of the present invention.

FIG. 7 is a system diagram showing a fifth embodiment of an oxygenisotope concentration device of the present invention.

FIG. 8 is a system diagram showing a sixth embodiment of an oxygenisotope concentration device of the present invention.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

11: Ozone formation unit

12: Ozone separation unit

13: Ozone photodissociation unit

14: Oxygen separation unit

15: Second ozone photodissociation unit

16: Second oxygen separation unit

21: Ozonizer

22: First distillation tower

23: Condenser

24: Reboiler

25: Heat exchanger

26: Buffer tank

27: Blower

31: Light source

32: Photoreaction cell

33: Second distillation tower

34: Condenser

35: Reboiler

36: Heat exchanger

51-56: Lines

57: Valve

58: Line

59: Valve

61-64: Lines

111: Ozone formation unit

112: Ozone separation unit

113: Ozone photodissociation unit

114: Oxygen isotope concentration unit

115-116: Lines

117-119: Rare gas feed lines

121: Second ozone photodissociation unit

122: Second oxygen isotope concentration unit

123: Ozone decomposition unit

124: Rare gas recovery unit

125-126: Lines

131: Line

211: Purifier

212: Photoreaction cell

213: Cold trap

214: Vacuum pump

215: Distiller

221-228: Lines

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a system diagram showing a first embodiment of the oxygenisotope concentration device of the present invention. Thisconcentration device is provided an ozonizer 21 for obtaining ozone fromraw material oxygen, a first distillation tower 22 that separates theozone formed with this ozonizer 21 and the raw material oxygen, acondenser 23 for imparting the cold required by the low-temperaturedistillation procedure in this first distillation tower 22, a reboiler24 for generating rising gas in the first distillation tower, a heatexchanger 25 for recovering the cold of the first distillation towereffluent gas in the first distillation tower feed gas, a buffer tank fortemporarily storing the first distillation tower effluent gas, and ablower 27 for circulating and feeding first distillation tower effluentgas in buffer tank 26 to ozonizer 21.

The equipment for concentrating oxygen containing a specific isotopefrom the ozone is provided with a photoreaction cell 32 for separating aspecific ozone isotopologue into oxygen by light radiated from lightsource 31, a second distillation tower 33 that separates the oxygenformed in photoreaction cell 32 from non-dissociated ozone toconcentrate the oxygen isotope in the oxygen, a condenser 34 forimparting the cold required by the low-temperature distillationprocedure in second distillation tower 33, a reboiler 35 for generatingrising gas in the second distillation tower, and a heat exchanger 36 forrecovering the cold of the second distillation tower effluent gas in thesecond distillation tower feed gas.

A portion of the raw material oxygen, which has been supplied from line51 and has merged with circulating oxygen from line 52, is ozonized bysilent discharge in ozonizer 21 to become an ozone-oxygen mixed gas, andafter this gas is cooled in heat exchanger 25, it is fed to theintermediate stage of first distillation tower 22 from line 53. Thisozone-oxygen mixed gas is distilled by a refluxing liquid formed withcondenser 23 located above the first distillation tower and a rising gasformed in reboiler 24 located beneath the first distillation tower, andthe liquefied ozone of the oxygen gas in the upper portion of firstdistillation tower 22 respectively separates in the bottom of firstdistillation tower 22. A portion of the oxygen that has been dischargedto line 54 from the first distillation tower branches to condenser 23,while the remainder is temporarily stored in buffer tank 26 afterpassing through line 55, after which it is compressed with blower 27 andcirculated and fed into ozonizer 21 from line 52.

A portion of the ozone that has been discharged from the bottom of firstdistillation tower 22 to line 56 branches to reboiler 24, while theremainder is fed to photoreaction cell 32 in a gaseous state afterpassing through valve 57. The specific isotopologue present in thisozone is dissociated into oxygen by light radiated from light source 31,and the ozone-oxygen mixed gas composed of dissociated oxygen andnon-dissociated ozone is led out from line 58 through valve 59. Insidephotoreaction cell 32, in order to carry out dissociation of a specificisotopologue of ozone efficiently in a stable state, together withreducing the pressure to 13 kPa or less, the inside of the photoreactioncell is cooled to within a range of 100-250 K. Pressure and temperaturemay be suitably set according to the dissociation status of the ozone,and at least the area between photoreaction cell 32 and valves 57 and 59on both sides should be maintained at a predetermined pressure andpredetermined temperature within ranges that do not cause the ozone toliquefy or solidify.

The ozone-oxygen mixed gas of line 58 is fed into the intermediate stageof second distillation tower 33 after being cooled with heat exchanger36, and similar to the aforementioned first distillation tower 22, isdistilled by a refluxing liquid from condenser 34 and a rising gas fromreboiler 35, causing ozone to separate in the lower portion of the towerand oxygen gas containing a specific oxygen isotope to concentrate inthe upper portion of the tower. The concentrated oxygen gas containing aspecific isotope is discharged from the upper portion of seconddistillation tower 33 into line 61, and after a portion of the gas hasbranched to condenser 34, is recovered in the form of a product fromline 62. After a portion of the ozone that has been discharged from thebottom of second distillation tower 33 into line 63 has branched toreboiler 35, the remainder is removed to line 64 after passing throughheat exchanger 36.

The ozone of line 64 is normally discharged after being dissociated intooxygen by an ozone photodissociation unit using a catalyst and so forth.The ozone of this line 64 can also be fed into a second photoreactioncell (not shown) serving as a second photodissociation unit providedseparately from the aforementioned photoreaction cell 32, anisotopologue can be dissociated that differs from that dissociated inphotoreaction cell 32, and the isotope-containing oxygen formed by thisdissociation can be separated from the ozone and concentrated by using asecond oxygen separation unit in the form of a distillation tower and soforth.

Although the operating conditions of each distillation tower arearbitrary, since the concentration of the desired isotope-containingoxygen gas decreases if oxygen enters the photoreaction cell, it ispreferable obtain ozone in a state that is as free of oxygen aspossible. Nitrogen or argon at a suitable temperature can be used forthe cooling source supplied to the condenser and the-heating sourcesupplied to reboiler. Nitrogen or argon at a suitable temperature canalso be used for cooling the photoreaction cell. Reduction of pressureinside the system that contains the photoreaction cell can be carriedout by installing a vacuum pump in a suitable line downstream from thephotoreaction cell or by reducing the pressure by liquefaction usingliquid nitrogen and so forth. A material that does not exhibitreactivity or catalytic action with ozone should be selected for thematerials of the equipment, and normally glass or fluororesin((polytetrafluoroethylene) and so forth is used preferably.

The calculated values of process flow rates and so forth in each linewhen producing 10 kg (as H₂O) annually by concentrating ¹⁷O using aconcentration device having the configuration shown in FIG. 3 are shownin Table 1. Oxygen gas in which ¹⁷O and ¹⁸O have been concentrated bydistillation is used for the raw material oxygen. ¹⁶O¹⁷O¹⁸O was selectedfor the isotopologue used for the purpose of degrading in thephotoreaction cell. Laser light having a wavelength of 992 nm was usedfor the light for degrading this isotopologue. The laser output was setto 1.0 W, and the absorption cross-sectional area was set to 3.0 ×10⁻²³cm². The pressure in the photoreaction cell was 13 kPa (100 Torr), andthe temperature was 200 K. The optical path length was 40 m, theretention time was 100 seconds, the light utilization rate was 0.0019,the yield of the target isotopologue was 0.90, and the amount ofnon-selective dissociation of other isotopologues generated simultaneousto dissociation of the target isotopologue was 3.3 with respect to avalue of 1 for the target isotopologue. The concentration rate of ¹⁷O atthis time was 10.8. The amount of power consumed by the ozonizer was 3.0kW.

TABLE 1 Line Number 51 52 53 55 56 58 64 62 Flow Rate No. mol/s 5.50E−043.85E−03 3.87E−03 3.30E−03 3.87E−03 3.69E−04 3.61E−04 8.42E−06 MolFraction 1 O₂ [−] 1.000 1.000 0.900 1.000 0.000 0.023 0.000 1.000 2 O₃[−] 0.000 0.000 0.100 0.000 1.000 0.977 1.000 0.000 Total 1.000 1.0001.000 1.000 1.000 1.000 1.000 1.000 Atom Fraction 1 ¹⁶O [−] 0.430 0.4300.430 0.430 0.430 0.430 0.4304 0.4026 2 ¹⁷O [−] 0.010 0.010 0.010 0.0100.010 0.010 0.0086 0.1015 3 ¹⁸O [−] 0.560 0.560 0.560 0.560 0.560 0.5600.5610 0.4959 Total 1.000 1.000 1.000 1.000 1.000 1.000 1.0000 1.0000Mol Fraction 1 ¹⁶O¹⁶O [−] 0.1849 0.1849 0.1664 0.1849 0.0037 0.1621 2¹⁶O¹⁷O [−] 0.0088 0.0088 0.0077 0.0088 0.0019 0.0817 3 ¹⁶O¹⁸O [−] 0.48160.4816 0.4334 0.4816 0.0091 0.3993 4 ¹⁷O¹⁷O [−] 0.0001 0.0001 0.00010.0001 0.0002 0.0103 5 ¹⁷O¹⁸O [−] 0.0112 0.0112 0.0101 0.0112 0.00230.1007 6 ¹⁸O¹⁸O [−] 0.3136 0.3136 0.2822 0.3136 0.0056 0.2459 7¹⁶O¹⁶O¹⁶O [−] 0.0080 0.0795 0.0780 0.799 8 ¹⁶O¹⁶O¹⁷O [−] 0.0004 0.00370.0036 0.0037 9 ¹⁶O¹⁷O¹⁶O [−] 0.0002 0.0018 0.0018 0.0019 10 ¹⁶O¹⁶O¹⁸O[−] 0.0207 0.2071 0.2033 0.2080 11 ¹⁶O¹⁸O¹⁶O [−] 0.0104 0.1035 0.10160.1040 12 ¹⁶O¹⁷O¹⁷O [−] 0.0000 0.0001 0.0001 0.0001 13 ¹⁷O¹⁶O¹⁷O [−]0.0000 0.0000 0.0000 0.0000 14 ¹⁶O¹⁷O¹⁸O [−] 0.0005 0.0048 0.0004 0.000415 ¹⁷O¹⁶O¹⁸O [−] 0.0005 0.0048 0.0047 0.0048 16 ¹⁶O¹⁸O¹⁷O [−] 0.00050.0048 0.0047 0.0048 17 ¹⁷O¹⁷O¹⁷O [−] 0.0000 0.0000 0.0000 0.0000 18¹⁶O¹⁸O¹⁸O [−] 0.0270 0.2697 0.2647 0.2709 19 ¹⁸O¹⁶O¹⁸O [−] 0.0135 0.13480.1324 0.1354 20 ¹⁷O¹⁷O¹⁸O [−] 0.0000 0.0001 0.0001 0.0001 21 ¹⁷O¹⁸O¹⁷O[−] 0.0000 0.0001 0.0001 0.0001 22 ¹⁷O¹⁸O¹⁸O [−] 0.0006 0.0063 0.00620.0063 23 ¹⁸O¹⁷O¹⁸O [−] 0.0003 0.0031 0.0031 0.0031 24 ¹⁸O¹⁸O¹⁸O [−]0.0176 0.1756 0.1724 0.1764 Total 1.00000 1.00000 1.00000 1.000001.00000 1.00000 1.00000 1.00000

Next, the calculated values of process flow rates and so forth in eachline in the case of producing 10 kg (as H₂O) annually using high-purityoxygen for the raw material oxygen are shown in Table 2 (Japanese PatentApplication No. 2003-57439, Table 2). ¹⁶O¹⁶O¹⁷O was selected for thetarget isotopologue. Laser light having a wavelength of 922 nm was usedas the light for degrading this isotopologue. The laser output was setto 2.2 W, and the absorption cross-sectional area was set to 3.0 ×10⁻²³cm². The pressure in the photoreaction cell was 13 kPa (100 Torr), andthe temperature was 150 K. The optical path length was 40 m, theretention time was 100 seconds, the light utilization rate was 0.0003,the yield of the target isotopologue was 0.90, and the amount ofnon-selective dissociation of other isotopologues generated simultaneousto dissociation of the target isotopologue was 10 with respect to avalue of 1 for the target isotopologue. The concentration rate of ¹⁷O atthis time was 85.4. The amount of power consumed by the ozonizer was 6.7kW.

TABLE 2 Line Number 51 52 53 55 56 58 64 62 Flow Rate No. mol/s 1.22E−038.54E−03 8.13E−03 7.32E−03 8.13E−04 8.16E−04 8.07E−04 8.89E−06 MolFraction 1 O₂ [−] 1.000 1.000 0.900 1.000 0.000 0.011 0.000 1.000 2 O₃[−] 0.000 0.000 0.100 0.000 1.000 0.989 1.000 0.000 Total 1.000 1.0001.000 1.000 1.000 1.000 1.000 1.000 Atom Fraction 1 ¹⁶O [−] 0.997590.99759 0.99759 0.99759 0.99759 0.099759 0.99781 0.98751 2 ¹⁷O [−]0.00037 0.00037 0.00037 0.00037 0.00037 0.00037 0.00015 0.03084 3 ¹⁸O[−] 0.00204 0.00204 0.00204 0.00204 0.00204 0.00204 0.00204 0.00185Total 1.00000 1.00000 1.00000 1.00000 1.00000 1.00000 1.00000 1.00000Mol Fraction 1 ¹⁶O¹⁶O [−] 0.99519 0.99519 0.89567 0.99519 0.010200.93607 2 ¹⁶O¹⁷O [−] 0.00074 0.00074 0.00066 0.00074 0.00065 0.05929 3¹⁶O¹⁸O [−] 0.00407 0.00407 0.00368 0.00407 0.00004 0.00359 4 ¹⁷O¹⁷O [−]0.00000 0.00000 0.00000 0.00000 0.00001 0.00094 5 ¹⁷O¹⁸O [−] 0.000000.00000 0.00000 0.00000 0.00000 0.00011 6 ¹⁸O¹⁸O [−] 0.00000 0.000000.00000 0.00000 0.00000 0.00000 7 ¹⁶O¹⁶O¹⁶O [−] 0.09928 0.99279 0.982630.99345 8 ¹⁶O¹⁶O¹⁷O [−] 0.00007 0.00074 0.00007 0.00007 9 ¹⁶O¹⁷O¹⁶O [−]0.00004 0.00037 0.00036 0.00037 10 ¹⁶O¹⁶O¹⁸O [−] 0.00041 0.00406 0.004020.00406 11 ¹⁶O¹⁸O¹⁶O [−] 0.00020 0.00203 0.00201 0.00203 12 ¹⁶O¹⁷O¹⁷O[−] 0.00000 0.00000 0.00000 0.00000 13 ¹⁷O¹⁶O¹⁷O [−] 0.00000 0.000000.00000 0.00000 14 ¹⁶O¹⁷O¹⁸O [−] 0.00000 0.00000 0.00000 0.00000 15¹⁷O¹⁶O¹⁸O [−] 0.00000 0.00000 0.00000 0.00000 16 ¹⁶O¹⁸O¹⁷O [−] 0.000000.00000 0.00000 0.00000 17 ¹⁷O¹⁷O¹⁷O [−] 0.00000 0.00000 0.00000 0.0000018 ¹⁶O¹⁸O¹⁸O [−] 0.00000 0.00001 0.00001 0.00001 19 ¹⁸O¹⁶O¹⁸O [−]0.00000 0.00000 0.00000 0.00000 20 ¹⁷O¹⁷O¹⁸O [−] 0.00000 0.00000 0.000000.00000 21 ¹⁷O¹⁸O¹⁷O [−] 0.00000 0.00000 0.00000 0.00000 22 ¹⁷O¹⁸O¹⁸O[−] 0.00000 0.00000 0.00000 0.00000 23 ¹⁸O¹⁷O¹⁸O [−] 0.00000 0.000000.00000 0.00000 24 ¹⁸O¹⁸O¹⁸O [−] 0.00000 0.00000 0.00000 0.00000 Total1.00000 1.00000 1.00000 1.00000 1.00000 1.00000 1.00000 1.00000

FIG. 4 is a system diagram showing a second embodiment of an oxygenisotope concentration device. It shows an example of the configurationof a device provided with equipment for obtaining a rare gas-ozone mixedgas in a stage prior to the concentration device.

This concentration device, together with being provided with an ozoneformation unit 111 that forms ozone from raw material oxygen GO, anozone separation unit 112 that separates raw material oxygen containingozone formed in said ozone formation unit 111 into ozone OZ and rawmaterial oxygen RO, an ozone photodissociation unit 113 that selectivelydegrades ozone containing a specific oxygen isotope in its molecule intooxygen by irradiating the ozone OZ separated in said ozone separationunit 112 with light L of a specific wavelength, and an oxygen isotopeconcentration unit 114 that separates oxygen OC formed by thedissociation of ozone in said ozone photodissociation unit 113 fromnon-dissociated ozone OZ to concentrate a desired oxygen isotope in saidoxygen, is also provided with a line 115 that feeds raw material oxygeninto ozone formation unit 111, a line 116 that feeds ozone-containingoxygen formed in ozone formation unit 111 into ozone separation unit112, or rare gas feed lines 117, 118 and 119 for feeding at least onetype of rare gas selected from helium, neon, argon, krypton, xenon andradon to at least one suitable location of ozone separation unit 112that concentrates ozone for use as a rare gas RG for dilution of ozone.

In rare gas feed line 117 among the aforementioned three rare gas feedlines 117, 118 and 119, a rare gas other than xenon and radon, for whichthere is the risk of oxidation or disintegration in ozone formation unit111 (ozone formation step), is mixed alone or as a plurality of typesand added to the raw material oxygen. In rare gas feed line 118, it ispreferable that a rare gas be added that can be supplied to ozonephotodissociation unit 113 together with ozone separated in ozoneseparation unit 112 (ozone separation step), namely a rare gas that isconcentrated on the ozone side in the distillation separation step andlow-temperature adsorption step.

Helium, neon or argon may also be added in consideration of the ease ofoperation of ozone separation unit 112. Since rare gas feed line 119 isfor feeding a rare gas for obtaining the rare gas-ozone mixed gassupplied to ozone photodissociation unit 113, a rare gas that isconcentrated on the ozone side in ozone separation unit 112 (at leastone type consisting of krypton, xenon and radon) is fed by this line.

In this manner, by carrying out an ozone photodissociation step in whicha rare gas-ozone mixed gas having a low ozone concentration is suppliedto ozone photodissociation unit 113 and irradiated with light of aspecific wavelength to selectively degrade an isotopologue of the ozonethat contains a specific oxygen isotope in its molecule into oxygen byphotodissociation, loss of ozone by spontaneous dissociation anddissociation of ozone by collision with formed oxygen can be suppressed,thereby making it possible to efficiently obtain oxygen containing aspecific oxygen isotope.

In the oxygen isotope concentration unit 114 (oxygen isotopeconcentration step), since the aforementioned rare gases such askrypton, xenon and radon are concentrated on the ozone side in the samemanner as ozone separation unit 112 when oxygen separated from ozone inozone photodissociation unit 113 is separated from non-dissociatedozone, there is hardly any concentration of these rare gases on theoxygen side, thereby allowing the obtaining of oxygen containing aspecific oxygen isotope at a high concentration.

A device may also be composed with only ozone photodissociation unit 113and oxygen isotope concentration unit 114 without providing ozoneformation unit 111 and ozone separation unit 112 by producing ozonepremixed with at least one type of rare gas consisting of krypton, xenonand radon, and feeding this rare gas-ozone mixed gas into ozonephotodissociation unit 113.

FIG. 5 is a system diagram showing a third embodiment of a concentrationdevice of the present invention. In the following explanation, the samereference symbols are used to indicate those constituents that are thesame as those in the concentration device shown in the previouslydescribed second embodiment, and their detailed explanations areomitted.

This oxygen isotope concentration device is provided with an ozoneformation unit 111, which performs silent discharge or radiation with amercury lamp and so forth on a rare gas-raw material oxygen mixed gas inthe form of a mixture of raw material oxygen GO fed from line 115,krypton RG circulated and fed from rare gas feed line 117, andcirculating raw material oxygen RO circulated and fed from line 126, anozone separation unit 112, which separates the rare gas-ozone- rawmaterial oxygen mixed gas fed from said ozone formation unit 111 intoline 116 into raw material oxygen RO that circulates to line 126 andrare gas-ozone mixed gas OR supplied to an ozone photodissociation unit113, ozone photodissociation unit 113, which selectively degrades aspecific isotopologue of ozone into oxygen by irradiating the raregas-ozone mixed gas OR separated in said ozone separation unit 112 withlight L of a specific wavelength, an oxygen isotope concentration unit114, which separates oxygen OC1 formed by dissociation of ozone in saidozone photodissociation unit 113 from rare gas-ozone mixed gas OR1composed of non-dissociated ozone and rare gas to concentrate theaforementioned oxygen isotope present in the separated oxygen OC1, asecond ozone photodissociation unit 121, which selectively degrades anisotopologue different from the isotopologue of ozone used in theaforementioned ozone photodissociation unit 113 into oxygen byirradiating the rare gas-ozone mixed gas OR1 separated in said oxygenisotope concentration unit 114 with light L2 of a different wavelengththan the light L1 used in the aforementioned ozone photodissociationunit 113, a second oxygen isotope concentration unit 122, whichseparates oxygen OC2 formed by dissociation of ozone in said secondozone photodissociation unit 121 from rare gas-ozone mixed gas OR2composed of non-dissociated ozone and rare gas to concentrate a specificoxygen isotope in the aforementioned oxygen OC2, an ozone decompositionunit 123 for decomposing the ozone contained in rare gas-ozone mixed gasOR2 separated in said second oxygen isotope concentration unit 122 bydegrading into oxygen, a rare gas recovery unit, which separates therare gas-oxygen mixed gas OR3 of oxygen and rare gas fed from said ozonedecomposition unit 123 into oxygen WO and rare gas RG to recover theseparated rare gas in the aforementioned rare gas feed line 117, a line125 for replenishing rare gas to rare gas feed line 117, and a line 126that circulates and feeds circulating raw material oxygen RO separatedin the aforementioned ozone separation unit 112 to the aforementionedozone formation unit 111.

It is preferable to use a distillation tower that separates oxygen andrare gas-ozone mixed gas by a low-temperature distillation separationstep for the aforementioned ozone separation unit 112, oxygen isotopeconcentration unit 114 and second oxygen isotope concentration unit 122.Although the operating conditions of each distillation tower arearbitrary, the ozone side should preferably contain as little oxygen aspossible. In addition, nitrogen, argon or oxygen at a suitabletemperature can be used for the cold source supplied to the condenserand heating source supplied to the reboiler.

A photoreaction cell provided with a specific light source can be usedfor ozone photodissociation unit 113 and second ozone photodissociationunit 121, and nitrogen, argon or oxygen at a suitable temperature can beused for the cold source when cooling the photoreaction cell. Whenreducing the pressure inside the system that contains the photoreactioncell, this can be carried out by installing a vacuum pump in a suitableline downstream from the photoreaction cell or by reducing the pressureby liquefaction using liquid nitrogen and so forth.

Heat dissociation or catalytic dissociation can be used in ozonedecomposition unit 123, which dissociates the entire amount of residualozone. A distillation tower using low-temperature distillation or anadsorption separation device using an adsorbent can be used for rare gasrecovery unit 124. A material that is not reactive or have catalyticaction on the ozone, such as glass or fluororesin(polytetrafluoroethylene), is preferably used for the material of theequipment.

Raw-material oxygen GO that is supplied from line 115 is fed into ozoneformation unit 111 in a state resulting from the merger of rare gas(krypton) RG supplied from rare gas feed line 117 and circulating rawmaterial oxygen RO supplied from line 126. A portion of raw material gasGO ozonized by silent discharge in ozone formation unit 111 in the formof an ozonizer and so forth, resulting in the formation of a raregas-ozone-raw material oxygen mixed gas that is fed into ozoneseparation unit 112. In the case of using a distillation tower for ozoneseparation unit 112, the mixed gas is fed into the intermediate stage ofthe distillation tower after cooling to a predetermined temperature witha heat exchanger. The rare gas-ozone-raw material oxygen mixed gas thathas entered the distillation tower is distilled resulting inconcentration of oxygen in the upper portion of the tower andconcentration of ozone and rare gas in the bottom portion of the towerby a low-temperature distillation separation step in the distillationtower. The oxygen that has been concentrated in the upper portion of thetower becomes circulating raw material oxygen RO which then circulatesto the upstream side of ozone formation unit 111 after passing throughline 126.

Rare gas-ozone mixed gas OR that has been discharged from ozoneseparation unit 112 is fed into ozone photodissociation unit 113 in theform of a gas, a specific isotopologue in the ozone is dissociated bylight L1, and oxygen is formed according to the aforementioned reactionformulas (1) and (2). The inside of ozone photodissociation unit 113 ismade to be at a low temperature and low pressure (for example, 100-250 Kand 13 kPa or lower) to ensure that dissociation of a specific ozoneisotopologue can be carried out efficiently and in a stable state. Thetemperature and pressure can be suitably set corresponding to the ozonedissociation status within a range in which there is no liquefaction orsolidification of ozone or rare gas.

Since ozone discharged from ozone separation unit 112 is diluted withrare gas, the probability of spontaneous dissociation due to contactwith a metal surface having catalytic action is lowered. According tothe aforementioned reaction formulas (1) and (2), since the reaction bywhich three molecules of oxygen are generated from two molecules ofozone is an exothermic reaction, the oxygen molecules formed bydissociation statistically have a large amount of kinetic energy,thereby enabling ozone molecules to break down into oxygen as a resultof oxygen molecules colliding with ozone molecules. Since thisdissociation of ozone molecules by collision with oxygen moleculesoccurs non-selectively, although there is a possibility of a desiredoxygen isotope being contained in the oxygen resulting from dissociationof ozone molecules, that probability is extremely low, and oxygencontaining a desired oxygen isotope resulting from dissociation byirradiation with light L1 ends up being diluted. However, by mixing raregas into the ozone, since oxygen molecules having a large amount ofkinetic energy dissipate their kinetic energy by colliding with raregas, the probability of ozone molecules being dissociated as a result ofoxygen molecules colliding with ozone molecules can be decreased.Consequently, the generation of oxygen not containing a desired oxygenisotope can be suppressed, thereby increasing the concentration rate ofthe desired oxygen isotope.

The rare gas-ozone-oxygen mixed gas containing oxygen dissociated fromozone in ozone photodissociation unit 113 is separated into oxygen OC1and rare gas-ozone mixed gas OR1 by a separation procedure such aslow-temperature distillation in oxygen isotope concentration unit 114,resulting in a state in which oxygen containing a desired oxygen isotopein oxygen OC1 is concentrated. The rare gas-ozone mixed gas OR separatedin oxygen isotope concentration unit 114 is irradiated with light L2having a wavelength that differs from that of the aforementioned lightL1 in second ozone photodissociation unit 121, resulting in dissociationof a desired isotopologue in the ozone into oxygen.

Rare gas-ozone-oxygen mixed gas that has been fed into second oxygenisotope concentration unit 122 from second ozone photodissociation unit121 is separated into oxygen OC2 in which oxygen containing a desiredoxygen isotope has been concentrated, and rare gas-ozone mixed gas OR2by a separation procedure such as low-temperature distillation in secondoxygen isotope concentration unit 122. This rare gas-ozone mixed gas OR2is subjected to ozone dissociation and decomposition treatment by beingfed into ozone decomposition unit 123, resulting in the formation of arare gas-oxygen mixed gas OR3 composed of oxygen resulting fromdissociation of ozone and rare gas, which is then fed into rare gasrecovery unit 124.

In rare gas recovery unit 124, an operation is carried out forseparating oxygen and rare gas, and waste oxygen WO separated from raregas-oxygen mixed gas OR3 is discharged outside the system, while theseparated rare gas RG is recovered in the aforementioned rare gas feedline 117, and then circulated and fed into raw material oxygen fed fromthe aforementioned line 115. In addition, since a portion of the raregas is lost in the separation procedure and so forth, a predeterminedamount of rare gas RG is replenished from line 125 so that a fixedamount of rare gas circulates within the system. In this manner, costscan be reduced by recycling the rare gas.

A rare gas feed line 118 similar to the second embodiment shown in theaforementioned FIG. 4 is provided in line 116 for the rare gas-ozone-rawmaterial oxygen mixed gas discharged from ozone formation unit 111, andat least one type of krypton, xenon and argon supplied to ozonephotodissociation unit 113 with ozone by ozone separation unit 112 canbe fed from this rare gas feed line 118, or at least one type of helium,neon and argon can be fed for improving the operation of ozoneseparation unit 112. Here, since helium, neon or argon fed from rare gasfeed line 118 concentrates on the raw material oxygen side in ozoneseparation unit 112, and then circulates through line 126 together withcirculating raw material oxygen RO, the amounts of these gases that arefed is about equal to the lost amount that is replenished after theamount of rare gas circulating through line 126 has reached a fixedamount.

FIG. 6 is a system diagram showing a fourth embodiment of aconcentration device of the present invention. This fourth embodimentmixes ozone by feeding at least one type of rare gas RG selected fromkrypton, xenon and radon into ozone separation unit 112 from rare gasfeed line 119. In this manner, by feeding rare gas on the downstreamside from ozone generation unit 111, the conversion of xenon or radoninto unstable oxides in ozone formation unit 111 can be prevented. Here,other aspects of the device configuration are the same as those of theaforementioned third embodiment.

FIG. 7 is a system diagram showing a fifth embodiment of a concentrationdevice of the present invention. The present embodiment feeds at leastone type of rare gas KG selected from helium, neon and argon into amixed gas containing oxygen formed in ozone photodissociation unit 113,non-dissociated ozone and rare gas from line 131 before feeding intooxygen isotope concentration unit 114. Krypton RG from rare gas feedline 117 is recycled and fed thereinto, as in the third embodiment.

Thus, oxygen generated by dissociation of ozone, non-dissociated ozone,krypton RG circulating within the system, and this rare gas KG composedof at least one type selected from helium, neon and argon can be fedinto oxygen isotope concentration unit 114 in a mixed state. At leastone type of rare gas KG selected from helium, neon and argon isseparated into high boiling point ozone and krypton by a separationprocedure such as low-temperature distillation in oxygen isotopeconcentration unit 114, and discharged with oxygen OC1 having a lowerboiling point. Thus, although oxygen containing a specific oxygenisotope is obtained in a state that is diluted with rare gas, since theflow rate can be regulated more easily as compared with a small amountof high-purity oxygen, handling becomes easier. Here, other aspects ofthe device configuration are the same as those of the aforementionedthird embodiment.

Similarly, although not shown in the drawings, line for feeding at leastone type of rare gas selected from helium, neon and argon may beprovided in a stage before second oxygen isotope concentration unit 122.This is similar to the fourth embodiment shown in FIG. 6.

In the fifth embodiment of the present invention, the peroxide used forthe raw material is a substance that has an “O—O” bond its molecule, andis any of the various organic peroxides represented by the chemicalformulas shown in Table 3, nitrous ester such as ONOCH₃, or nitrateester such as O₂NOCH₃. In Table 3, R and R′ respectively represent ahydrogen atom or alkyl group, and R and R′ may be the same or different.

TABLE 3 Chemical Formula Name ROOH hydroperoxide ROOR′ (di)alkylperoxide RC(═O)OOH peracid, peroxy acid RC(═O)OOC(═O)R′ (di)acylperoxide RC(═O)OOR′ perester, peroxy ester ROOC(═O)OR′ peroxycarbonateROC(═O)OOC(═O)OR′ peroxydicarbonate ROOC(═O)OR′ diperoxycarbonateROOC(═O)—C(═O)OOR′ per(oxy)oxalate

cyclic peroxide, ozonide, endoperoxide

The bond dissociation energies of each of the aforementioned compoundsare described in Table 4 of The Chemical Record (2nd Edition, BasicChemistry Edition, The Chemical Society of Japan, published Jun. 20,1975, p. 978). In Table 4, the values of bond dissociation energies areindicated as the values of those bonds indicated with hyphens “-”.

TABLE 4 Substance kJ/mol NN—O 167.0 ON—OCH₃ 152.3 ON—OC₂H₅ 157.7ON—OC₃H₇ 157.7 O₂N—OCH₃ 160.7 O₂N—OC₂H₅ 152.3 O₂N—OC₃H₇ 156.5(CH₃)₃CO—OH 163.6 C₂H₅O—OC₂H₅ 132.2 (CH₃)₃CO—OC(CH₃)₃ 155.0CH₃COO—OCOCH₃ 123.4 C₂H₅COO—OCOC₂H₅ 126.0 C₃H₇COO—OCOC₃H₇ 126.0C₆H₅COO—OCOC₆H₅ 129.7

The values shown in Table 5 are obtained when the bond dissociationenergies of the substances shown in Table 4 are converted to the wavenumber and wavelength of light based on the correlation that 1J/mol=0.083593462 cm⁻¹.

TABLE 5 Substance cm⁻¹ μm NN—O 13960 0.716 ON—OCH₃ 12731 0.785 ON—OC₂H₅13183 0.759 ON—OC₃H₇ 13183 0.759 O₂N—OCH₃ 13433 0.744 O₂N—OC₂H₅ 127310.785 O₂N—OC₃H₇ 13082 0.764 (CH₃)₃CO—OH 13676 0.731 C₂H₅O—OC₂H₅ 110510.905 (CH₃)₃CO—OC(CH₃)₃ 12957 0.772 CH₃COO—OCOCH₃ 10315 0.969C₂H₅COO—OCOC₂H₅ 10533 0.949 C₃H₇COO—OCOC₃H₇ 10533 0.949 C₆H₅COO—OCOC₆H₅10842 0.922

As can be seen from Table 5, these compounds are able to undergo earlydissociation by absorbing light within the range of visible light tonear infrared light. Namely, semiconductor lasers that radiate at awavelength in the visible to near infrared bands can be used as a lightsource, examples of which include InGaAsP, AlGaInP, GaAsSb, CdZnTe,CdZnSe, AlGaN and InGaN semiconductor lasers.

When focusing on the —COO—O— bond, the wavelength required fordissociation of this bond can be seen to be 0.9-1.0 μm. In particular,peracids and peroxyacids having a —COO—O— bond in their molecules arepreferably used in the present invention due to the low toxicity of theraw material, reaction products and so forth, and acetic peracid(CH₃COOOH)is particularly preferable. This is because this substance hasa —COO—O— bond in its molecule, the optical absorption spectrum containsa vibration mode for the C═O bond, and this substance can be dissociatedby irradiating with laser light having a wavelength that matches thevibration mode of C═¹⁷O or C═¹⁸O. Since acetic peracid also has avibration mode for the O—H bond in addition to the vibration mode forthe C═O bond, irradiating with laser light at a wavelength that matchesthe vibration mode of ¹⁷O—H or ¹⁸O—H makes it possible selectivelydegrade them.

The photodissociation reaction in the case of using a hydroperoxide(ROOH) containing ¹⁷O—H for the peroxide becomes as shown in thefollowing reaction formulas (1a) through (1d). Furthermore, the blackdots in each reaction formula represents radicals. In addition, althoughthe example of ¹⁷O is used for the oxygen isotope in the followingexplanation, the explanation applies similarly to ¹⁸O.

In reaction formulas (1b) through (1d), since large amounts of radicals“RO.” and “ROO.” are formed, it is preferable to suppress thesereactions by diluting the peroxide with solubilizing solvent such as atleast one type of solvent selected from carbon tetrachloride, acetone,acetic acid, hexane, toluene and chlorofluorocarbons.

The photodissociation reaction in the case of dialkyl peroxidecontaining ¹⁷OR′ is shown in the following reaction formula (2).

The photodissociation reaction in the case of peroxy acid containing¹⁷O—OH is as shown in the following reaction formulas (3a) and (3b).

The reaction shown in reaction formula (3b) is an exothermic reactionthat is considered to have a high reaction probability. In addition,since peracids and peroxy acids are easily dissociated by heat at high,concentrations and spontaneous degrade explosively in certain cases, itis preferable to dilute with a solvent as previously described tosuppress radical reactions.

FIG. 8 is a system diagram showing an example of the configuration of adevice for carrying out the oxygen isotope concentration method of thepresent invention. This oxygen isotope concentration device is providedwith a purifier 211, which removes impurities by purifying raw materialvapor in which peroxide is diluted with solvent, a photoreaction cell212, which radiates light of a specific wavelength onto the raw materialvapor following purification, a cold trap 213, which captures vapor forwhich the photodissociation reaction in said photoreaction cell 212 hasbeen completed by condensing or solidifying on a metal surface cooled toa low temperature, a vacuum pump 214 for reducing the pressure insidethe aforementioned photoreaction cell 212 to a low pressure such as apressure of 13 kPa or less, and a distiller 215 for separating eachcomponent captured in the aforementioned cold trap 213.

Peroxide serving as the raw material is diluted to a suitableconcentration with a solvent followed by vaporization and being fed intopurifier 211 from line 221. In this purifier 211, raw material peroxide,from which impurities and water, for example, have been removed, in thispurifier 211 is fed into photoreaction cell 212 through line 222. As aresult of being irradiated with laser light hv of a specific wavelengthin photoreaction cell 212, the peroxide undergoes the reactions ofreaction formulas (1b) through (1d) accompanying dissociation of aspecific bond in its molecule, for example, the RO—OH bond inhydroperoxide (ROOH) containing ¹⁷O—H as shown in the aforementionedreaction formula (1a). In this reaction, ¹⁷O is concentrated in watermolecules.

Vapor containing water molecules in which ¹⁷O is concentrated are fedfrom photoreaction cell 212 to cold trap 213 through line 223. This coldtrap 213 is cooled by a chiller unit and so forth to a temperature(e.g., −20 to −5° C.) that makes it possible for a metal surface tocapture the aforementioned water molecules. Since each of the componentsthat condense or solidify at this temperature are captured on the metalsurface of the chiller unit, gas such as oxygen that does condense orsolidify passes through this cold trap 213 and is discharged from vacuumpump 214 after passing through line 224.

In this stage, path 225 between cold trap 213 and distiller 215 isclosed by a valve and so forth.

After a suitable amount of raw material peroxide is allowed to passthrough, in addition to stopping vacuum pump 214, lines 223 and 224 infront of and in back of cold trap 213 are closed with valves and soforth, and after opening a valve and so on of line 225, normaltemperature nitrogen gas is fed at atmospheric pressure from line 226into cold trap 213, and in addition to returning the pressure insidecold trap 213 to atmospheric pressure, the temperature is raised tonormal temperature resulting in vaporization of each component capturedon the metal surface, which then are fed into distiller 215 form line225. In distiller 215, a distillation procedure is carried outcorresponding to the composition of the vapor that has entered, andtogether with removing water in which ¹⁷O is concentrated from one ofthe lines 227, unnecessary components are discharged from the otherlines. As a result, water in which ¹⁷O is concentrated is obtained asthe final product.

Even in cases in which ¹⁷O or ¹⁸O is concentrated in molecules otherthan water molecules, by appropriately setting the temperature of coldtrap 213 and the operating conditions of distiller 215, a desiredsubstance can be easily extracted as the final product. In addition, asuitable purification unit corresponding to the composition can beemployed for the purification method, and a purification method like gaschromatography can be employed.

More specifically, the following indicates the case of concentrating ¹⁷Oin water molecules by dissociating the (CH₃)₃CO—¹⁷OH bond in themolecules using (CH₃)₃COOH (tertial-butyl hydroperoxide), which is atype of hydroperoxide ROOH, for the peroxide that contains ¹⁷O. Thewavelength of the radiated laser light is 0.731 μm or less from theaforementioned Table 5. Since this wavelength band is the wavelengthrange that allows the use of an InGaAsP semiconductor laser, byprecisely matching to the wavelength capable of dissociating ¹⁷O, aspecific (CH₃)₃COOH containing ¹⁷O can be dissociated according to thephotodissociation reaction shown in the aforementioned reaction formula(1a).

In the case of C₂H₅OOC₂H₅ (diethyl peroxide), which is a type of dialkylperoxide ROOR′ and in which R and R′ are both ethyl groups, by radiatinglaser light of a wavelength shorter than 0.905 μm that matches thevibration mode of the O—¹⁷O bond or C—¹⁷O bond using, for example, anInGaAsP or InGaAs semiconductor laser, it can be dissociated as shown inreaction formula (2-1a) while specifying the C₂H₅OOC₂H₅ that containsthe oxygen isotope ¹⁷O. The radicals formed here form C₂H₅OH and CH₃CHOby going through the reaction process shown in the following reactionformulas (2-1b) through (2-1d), and ¹⁷O is concentrated in C₂H₅OH. TheC₂H₅O. in reaction formulas (2-1b) through (2-1d) also includes radicalsthat contain ¹⁷O.

In the case of CH₃OOCH₃ (dimethyl peroxide), in which both R and R′ ofthe dialkylperoxide are methyl groups, ¹⁷O can be concentrated in CH₃OHaccording to the following reaction formulas (2-2a) through (2-2c).

In the case of (CH₃)₃COOC(CH₃)₃ (dtBP: ditertial butyl peroxide), inwhich both R and R′ of the dialkyl peroxide are t-butyl groups, by usinga wavelength shorter than 0.772 μm, which is the wavelength range of anInGaAsP semiconductor laser, that is precisely matched to the vibrationmode of the O—¹⁷O or C—¹⁷O bond, it can be dissociated while specifyingdtBP containing the oxygen isotope ¹⁷O as shown in reaction formula(2-3a).

A list of all the reactions while ignoring intermediate reactionsresults in the following reaction formulas (2-3b-1), (2-3b-2-1) and(2-3b-2-2), and the ¹⁷O is concentrated in acetone. Here, (2-3b-1)accounts for about 90% of the product.

t-amyloxy radicals formed in the vapor phase photodissociation of t-amylethyl peroxide dissociate in two ways as shown in the following reactionformulas (2-4a) and (2-4b). The methyl radical CH₃. and ethyl radicalC₂H₅. formed here are stabilized by bonding with each other. Thus, ¹⁷Ois concentrated in acetone or methyl ethyl ketone.

In the case of acetic peracid, in which R is a methyl group in theperoxy acid RCOOOH (and including peracids), by using a wavelength inthe visible light band of 0.545-0.660 μm, which is the wavelength rangeover which an AlGaInP semiconductor laser can be used, that is preciselymatched to the vibration mode of the ¹⁷O—H bond, acetic peracidcontaining the oxygen isotope ¹⁷O can be selectively dissociatedaccording to the reaction formula shown in the aforementioned reactionformula (3a).

Here, a list of all reactions with respect to those reactions whentoluene was used as solvent results in the following reaction formula(3c), and the oxygen isotope¹⁷O is concentrated in molecules of water.However, although H₂ ¹⁷O is formed due to an elementary reaction withsolvent, the product relating to this solvent is ignored in reactionformula (3c).

In the aforementioned reaction formula (3c), y₁, y₂, y₃ and y₄ are about0.5, 0.4, 0.1 and 0.05, respectively, while y₅ and y₆ are trace amounts.In addition to the aforementioned toluene, those substances having adouble bond such as ethylene can be used as radical capturers.

As has been explained above, according to the present invention, byselecting a photodissociation reaction of ozone or peroxide as a meansof separating and concentrating an oxygen isotope, stable isotopes ofoxygen in the form of ¹⁷O and ¹⁸O can be concentrated efficiently.

The oxygen isotopes ¹⁷O and ¹⁸O concentrated by the method of thepresent invention can be used as tracers in the fields of chemistry andmedicine.

1. An oxygen isotope concentration method comprising: (a) an ozonephotodissociation step, in which ozone molecules containing oxygenisotopes, ¹⁷O and/or ¹⁸O, are selectively photodissociated to oxygenmolecules using a light in the near-infrared range of 700-1000 nm;thereby generating a mixture of the oxygen molecules and non-dissociatedozone molecules; and (b) an oxygen isotope concentration step, in whichthe oxygen molecules are separated from the mixture, therebyconcentrating the oxygen isotope.
 2. An oxygen isotope concentrationmethod according to claim 1, wherein at least one rare gas selected fromkrypton, xenon and radon is added in the step (a).
 3. An oxygen isotopeconcentration method according to claim 1, wherein the oxygen isotopeconcentration step (b) is a distillation carried out by adding at leastone rare gas selected from helium, neon and argon.
 4. An oxygen isotopeconcentration method according to claim 1, wherein the wavelength of thelight used in the ozone photodissociation step (a) is within the rangeof 991.965-992.457 nm.
 5. An oxygen isotope concentration methodaccording to claim 1, wherein the absorption wavelength of ozone isadjusted by applying an electric field in the ozone photodissociationstep (a).
 6. An oxygen isotope concentration method according to claim1, wherein the ozone photodissociation step (a) is carried out at lowtemperature and at low pressure.